This disclosure relates to a sensor, apparatus and method for non-invasively monitoring blood characteristics of a subject. The apparatus is typically a pulse oximeter, while the sensor is typically a pulse oximeter sensor attachable to a subject and adapted to acquire (photo) plethysmographic signals from the subject.
Plethysmography refers to measurement of changes in the sizes and volumes of organs and extremities by measuring changes in blood volume. Photoplethysmography relates to the use of optical signals transmitted through or reflected by blood for monitoring a physiological parameter of a subject. Conventional pulse oximeters use red and infrared photoplethysmographic (PPG) waveforms, i.e. waveforms measured respectively at red and infrared wavelengths, to determine oxygen saturation of pulsatile arterial blood of a subject. The two wavelengths used in a conventional pulse oximeter are typically around 660 nm (red wavelength) and 940 nm (infrared wavelength).
Pulse oximetry is at present the standard of care for continuous monitoring of arterial oxygen saturation (SpO2). Pulse oximeters provide instantaneous in-vivo measurements of arterial oxygenation, and thereby an early warning of arterial hypoxemia, for example. Pulse oximeters also display the photoplethysmographic waveform, which can be related to tissue blood volume and blood flow, i.e. the blood circulation, at the site of the measurement, typically in finger or ear.
Traditionally, pulse oximeters use the above-mentioned two wavelengths, red and infrared, to determine the oxygen saturation. Other parameters that may be determined in a two-wavelength pulse oximeter include pulse rate, peripheral perfusion index (PI) and pleth variability index (PVI), for example. Increasing the number of wavelengths to at least four allows the measurement of total hemoglobin (THb, grams per liter) and different hemoglobin types, such as oxyhemoglobin (HbO2), deoxyhemoglobin (RHb), carboxyhemoglobin (HbCO), and methemoglobin (metHb). A prerequisite of the measurement of total hemoglobin is that the wavelengths used extend up to a range where water absorption is high, thereby to be able to detect the concentrations of both hemoglobin and water. In practice, a pulse oximeter designed to measure total hemoglobin may be provided with 8 to 16 wavelengths (i.e. light sources) ranging from around 600 nm up to around 1300 nm.
The measurement of the blood characteristics is typically predicated on the assumption that the light beams from the different light sources follow identical paths through the intervening tissue to the photo detector. If this assumption is not made, the measurement becomes very complicated as the path lengths need to be determined for each wavelength. However, in multi-wavelength oximeters capable of determining total hemoglobin the use of multiple photo detectors becomes a necessity since there is no photo detector available that is capable of receiving such a wide range of wavelengths with acceptable reception characteristics. For example, the responsivity of widely used silicon photo detectors drops rather abruptly around 1000 nm, while modern InGaAs (indium gallium arsenide) photo detectors are sensitive from approximately 900 nm to approximately 1700 nm.
Consequently, the sensor of a multi-wavelength pulse oximeter is normally designed so that the light beams travel substantially along a common path through the tissue to be monitored, i.e. that the optical path length through the arteriolar bed is substantially the same for all wavelengths. As to the transmission end of the optical signals, it is normally not difficult to arrange the multiple small size light sources of the sensor in a substantially point-like fashion so that the optical path remains substantially the same for all wavelengths at the transmission end. However, it is more difficult to arrange two photo detectors, which have a rather large area, within the sensor so that the same requirement is fulfilled also at the reception end, thereby to avoid introduction of error into the measurement due to the inability of the sensor to transmit the light pulses along a common path at all wavelengths.
One solution for the above problem is to use a sandwich or layered detector design in the sensor. This involves that the photo detector consists of a multiple layer detector element that comprises two detector layers placed on top of each other. For example, a germanium photodiode may be placed under a silicon photodiode. This layered element operates so that for wavelengths under about 1000 nm the upper silicon photodiode receives the transmitted light. Above this wavelength, the silicon photodiode becomes substantially transparent and the lower germanium photodiode receives the light pulses.
A drawback of the sandwich or layered detector design is the rather complex mechanical structure that requires accuracy in the manufacturing process. These properties tend to translate into high costs for the end user, which in turn hampers the introduction and proliferation of the multi-wavelength measurements. In the sandwich design, the top detector also attenuates the light received by the bottom detector, which thus typically has a lower sensitivity than the detector exposed to direct light beams.